An and Ritsu were good friends in childhood. She is from a wealthy family like Ritsu, and both her and Ritsu's parents are good friends. This led to their parents arranging the marriage between the two of them.

An confesses her love.

An took the engagement seriously and fell in love with Ritsu, confessing to him when he was fifteen. However, Ritsu turned her down since he was in love with someone else, and requested that things remain the same between them. An agreed and the two remained good friends, although An continued to hope that Ritsu might someday return her affections.

While apart, An dated other guys, but none of the relationships lasted as she was still in love with Ritsu.

An does not appear in the first season, but Ritsu's fiance is referenced by Takafumi Yokozawa. When Ritsu and Takano come home from work very late, she is formally introduced while waiting for Ritsu in the cold at his apartment with gifts from France. She describes Takano as a friend and tells him to take care of Ritsu. She doesn't know that Ritsu and Takano had an argument which involves her after she left.

During Marukawa's New Years' Party, An's friend was having a wedding at the same hotel where Marukawa was having their party. After An realizes that Ritsu will be at the hotel, she insists him to meet her at the front lobby. Ritsu, being late due to the party, An gets angry at him asking if they were still engaged even after he rejected her. She informed him that while she dated other people, she realized that Ritsu is the only one she loves. An declared to Ritsu that even after he rejected her before, she still loves him. Ritsu sadly tell An that although he loves her too, his feelings aren't romantic. This devastated An and she asked Ritsu if he love someone else, especially the person he loves back in middle school. An informed Ritsu while she knows he dated other people, the relationships never last that long; she believed that Ritsu loves her but that wasn't the case. An now believed that he is still in love with his first love. Ritsu tried to rebuffed that statement but realized that this may be the case. Just when Ritsu told her otherwise, Takano interrupted them and tell him to go back to work. She finds out that Takano is not just his neighbor but also his boss. Upset from their conversation, An left the hotel in a hurry. Later that night, An called Ritsu's mother only to start crying midway from their conversation. This immediately upset Ritsu's mother and she angrily called Ritsu. She then demanded him to tell her what happened between them at the hotel.

Her first appearances are very much of that in the anime. She calls Ritsu to tell him that she has come home from France and has brought gifts. She waits for him outside his apartment building to see him and is unaware of the tension she caused between Takano and Ritsu. She later goes to the Marukawa New Years Party and waits for Ritsu to come see her. When he is unable to come until after the party ends, she is shown still waiting furiously. Ritsu explains how he told her he would be working and unable to show up.

An upset at the New Years Party.

She then confesses her love for Ritsu, which Takano overhears, who turns her down. When she brings up her theory that he is still in love with the person he loved in high school, he admits that might still be the case. When Takano interrupts the scene, she takes off in tears.
A few months later, An calls Ritsu telling him that his mother is at the hospital. When Ritsu expressed his gratitude, An immediately scolded that Ritsu was the reason why his mother collapsed. Since Ritsu never came home once since his transfer and mostly ignored his mother's calls, his mother was concerned. She expressed Ritsu her fears about not being able to do more; Ritsu comforted An and apologized for not thinking about her feelings. When Takano arrived at the hospital, Takano witnessed Ritsu comforting An and left; Ritsu shoved An away and chased after Takano to explain himself. When he was leaving, An witnessed Ritsu making a face he never usually make. (It was later noted that the first time she had seen Ritsu making that kind of 'face' or 'expression' was in high school which was when he was 'sort-of' going out/in love with Takano.)

On the way home, Ritsu gave An a drink and thank her for being there for his mother. An tells Ritsu that he's so kind which is one of the reason why she fell in love with him. An once again asked Ritsu if he could make her become her girlfriend. Ritsu sadly replied that he can't accept her request. An confronted Ritsu regarding his relationship with Takano and told him about the 'face' he makes only when he is with Takano. She is suspicious of Ritsu's true relationship with Takano and asked if the two are dating. Ritsu asked if she was crazy and she tells Ritsu that she knew that Ritsu's relationship with Takano was more then that of a friend and boss. Takano was revealed to actually be waiting for Ritsu outside the hospital in order to take Ritsu home and hears the portion of their conversation following when they went outside.

Meanwhile, Takano cuts in to reply to An's statement and she tells Takano that he better take care of Ritsu. She bids Ritsu well and leaves. A few months later, Ritsu later gets a call from his mother telling him that An officially canceled her engagement with Ritsu with the explanation that while she loves Ritsu as her friend, she cannot see or love him as her husband.

Ritsu is An's childhood friend and ex-fiancé. Though Ritsu never agreed to the engagement and repeatedly tried to call it off, to which An seems to agree with. She had confessed genuine feelings for him in the past, although he turned her down. An is aware that Ritsu has feelings for Masamune Takano after she realized that their relationship was more than just that of boss/subordinate and neighbors, this being after seeing the 'face' he only makes whenever he is with him. An is characterized as a very cheerful person, but can be stubborn to Ritsu.

She appears to have something of a good relationship with Ritsu's mother since she was the only one able to finally call off the arranged engagement between her and Ritsu, claiming that while she loves him as her friend, she cannot see him as her husband. During her "engagement" to Ritsu, she stated that she and Ritsu dated other people.

At first, An only considered Takano as Ritsu's neighbor and later learned he was also Ritsu's boss after running into him at Marukawa's New Years' Party when hoping to see Ritsu. Takano considered her a rival because of her cute appearance and how she seemed to have a close relationship with Ritsu, therefore fearing that she could easily steal him away.

After an incident with Ritsu's mother, An confronts Ritsu about his relationship with Takano, coming to the conclusion that the true nature of his relationship with Takano was more than just platonic. An then confronts Takano himself about his true relationship with Ritsu and gives her blessing, even brushing off the same-gender status, simply demanding that Takano take care of Ritsu and not to hurt him. She then finally cancelled her and Ritsu's engagement that their parents arranged, declaring that there is no romantic love between them.

After finding out about Ritsu having a romantic relationship with Takano, she did not judge him. When Ritsu asked her if two men having a relationship was unnatural, An simply said that she did not believe herself to be in a position to judge anyone for it and only wished for him to be happy. She even urged him to accept himself and that if he was going to be with Takano, then to make sure his feelings were not halfhearted.